[0001] The invention refers to a method of removably sealing the end of an Inconel heat
exchanger tube with a hollow Inconel plug.
Background of the invention
[0002] In tube-and-sheet heat exchangers, for example those used in a nuclear steam generator,
problems are encountered in the form of tubes which become defective or damaged. These
tubes are generally effectively removed from operation by plugging both ends of the
tubes during a maintenance shutdown and continuing the operation of the steam generator
with only the remaining tubes in operation. One means used in the past for plugging
the tubes was by welding a plug into each of the tube ends, as disclosed in US-A-4,262,187.
[0003] This works fine until a point is reached where there are so many tubes plugged that
there is insufficient capacity left for continued practical operation of the steam
generator. Then the unit is normally taken out of operation altogether and replaced
with an entire new steam generator. Since the cost of replacing an entire steam generator
is extremely high, other means of handling the situation is presently being considered.
One means is to plug the tubes by means of plugs which can be readily and easily removed
at a later time. Thus when too many tubes have been removed from operation, the unit
can be shut down, the plugs can be removed, and the tubes can be thoroughly inspected
and tested to see which tubes can be effectively repaired, for example by sleeving.
If enough tubes can be repaired, the useful life of the steam generator can be greatly
extended.
[0004] In GB-A-1,247,383, a method of plugging plastic tubes is disclosed using a rigid
plug and a flexible sleeve both made of plastics, the flexibility of the sleeve accommodating
any irregularities in the tube inner surfaces. In US-A-4,178,966, a method is disclosed
of plugging a defective metal tube with a metal plug, the plug having a first tapered
end and a second end which has a flange for limiting the insertion of the plug into
the tube hole and for providing filler material so as to be welded at the tube hole
opening.
Summary of the invention
[0005] The present invention is directed to a method of removably sealing the end of an
Inconel heat exchanger tube with a hollow Inconel plug, which plug has a substantially
non-varying outer diameter throughout the major longitudinal portion located within
the tube of slightly smaller diameter than that of the plug, said plug having a first
closed end which enters the tube, and a second open end adapted to be flush with the
tube end, a portion of the plug near the second end being of slightly larger diameter
to provide a light interference fit in the tube to hold the plug in place prior to
and during the expansion thereof, there being a band of ductile metal being plated
on a portion of the outer surface of the plug, said band being approximately 2-4 mils
(ca. 0,05-0,1 mm) thick and being of either gold, silver, or pure iron, including
the steps of inserting the plug into the tube until the portion of the plug of slightly
larger diameter comes into contact with, and forms a light interference fit in the
tube, with the open end of the plug being flush with the tube end, and thereafter
expanding the plug in the vicinity of the band of ductile metal, such that the plug
is expanded into tight engagement with the tube, and the ductile metal is extruded
into any crevices caused by irregularities in the inner wall surface of the tubes.
[0006] The expansion of the plug can be obtained by using mechanical rolls. Hydraulic or
pneumatic pressure could also possibly be used in place of the mechanical rolls. Many
times the inner wall of the tube has irregularities on its surface, such as longitudinal
scratch marks, which prevent a perfect seal. The thin band of ductile metal which
is provided on the tube plug is extruded into any crevices or irregularities, so as
to form a perfect seal.
Brief description of the drawing
[0007]
Figure 1 is a sectional elevation of a tube plug constructed in accordance with the
invention; and
Figure 2 is a partial sectional elevation of a tube-and-sheet heat exchanger showing
a tube plug expanded into place in the end of one of the tubes.
Description of the preferred embodiment
[0008] Looking now to Figure 1, numeral 10 indicates the tube plug in its entirety. This
plug can be used to removably seal the end of a defective or degraded tube in a nuclear
steam generator. These tubes are commonly made of an Inconel alloy, and the plug is
also of an Inconel alloy. The plug is closed at its inner end 12, which is the end
inserted into the degraded or defective tube. The tubes, and thus also the tube plugs,
are generally of small diameter (3/4 inch or 19,05 mm) as steam generating tubes go.
The plug 10 is approximately 5 inches (0,127 m) in length, and would be slightly less
than 3/4 of an inch (19,05 mm) OD throughout most of its length, if the degraded tube
had 3/4 of an inch (19,05 mm) ID. The outer end 14 ofthe plug should be slightly larger
than 3/ 4 of an inch (19,05 mm), to provide a light interference fit to hold the plug
in place prior to and during rolling. The plug will be rolled, or otherwise expanded,
for a length of approximately 1 1/4 inches (31,75 mm), shown at 16 in Fig. 1. In order
to provide a better seal between the plug and tube, a narrow band of ductile metal
is plated onto the tube plug, as indicated by numeral 18 in Fig. 1. This band may
be approximately 1/2 inch (12,7 mm) wide, and 2-4 mils (ca. 0,05-0,1 mm) thick. Suitable
ductile metals would be gold, silver, and pure iron. The portion 16 of the plug which
is expanded into the tube includes and overlaps on both sides of the band of ductile
metal 18. Thus during the plug expanding step of the process, the ductile metal is
extruded into all of the crevices caused by surface irregularities, such as scratches,
thus forming a positive seal. This is of the utmost importance in a nuclear steam
generator, since the primary fluid of the heat exchanger is radioactive, whereas the
secondary fluid is not. It is highly desirable to prevent intermixing of these fluids.
[0009] Figure 2 shows a tube plug 10 after it has been expanded into place in a defective
or degraded tube 20. Numeral 22 indicates the tube sheet of a nuclear steam generator
to which the ends of a plurality of U-shaped tubes 20 are secured. During operation
of the steam generator, some of these tubes become defective or degraded and must
be plugged. The plug 10 is positioned in the defective tube 20 with the plug end 14
flush with the face of tube sheet 22. The enlarged end 14 has a slight interference
with tube ID to hold the plug in place prior to and during the rolling procedure.
[0010] Any suitable rolling apparatus can be used for expanding the plug in place. A typical
mechanical tube expander may have a plurality of rolls held loosely in a cage or housing.
A tapered mandrel moves longitudinally through the housing, camming the rolls radially
outwardly into engagement with the walls of the plug. Further longitudinal movement
causes expansion of the plug. It should also be mentioned that an expanding device
could also be used which would utilize hydraulic or pneumatic pressure. The expansion
causes the ductile metal to be extruded and flow into any crevices caused by surface
irregularities, so that a good seal is formed. A plug such as described above is placed
in each end of every defective tube.
[0011] The tube plugging can be done during any maintenance shutdown of the steam generator.
The unit can thereafter be placed back into operation, with the plugged tubes remaining
out of operation as heat exchange surface. After a large number of tubes have been
plugged, the time may come when it is necessary to unplug some of the tubes and repair
them in order to continue to operate the steam generator effectively. The tube plugs
of the invention can be readily removed by heat shrinking. A heating device, such
as an electric induction device, can be inserted into the plug interior to heat the
metal to a high temperature. After cooling, the plug either slides out or can be pulled
out of the tube by a 10 to 20 pound force from a special gripping tool which grips
the plug by its ID wall surface. The tube can then be tested and sleeved if it is
repairable, so as to be operative as heat exchange surface when the unit is again
put back into operation.
Method of removably sealing the end of an Inconel heat exchanger tube (20) with a
hollow Inconel plug (10), which plug has a substantially non-varying outer diameter
throughout the major longitudinal portion located within the tube of slightly smaller
diameter than that of the tube, said plug having a first closed end (12) which enters
the tubes, and a second open end adapted to be flush with the tube end, a portion
of the plug near the second end being of slightly larger diameter to provide a light
interference fit in the tube to hold the plug in place prior to and during the expansion
thereof, a band of ductile metal (18) being plated on a portion of the outer surface
of the plug, said band being approximately (2-4 mils (ca. 0,05-0,1 mm) thick, and
being of either gold, silver, or pure iron, the method comprising the steps of inserting
the plug into the tube until the portion of the plug of slightly larger diameter comes
into contact with, and forms a light interference fit in the tube, with the open end
of the plug being flush with the tube end, and thereafter expanding the plug in the
vicinity of the band of ductile metal, such that the plug is expanded into tight engagement
with the tube, and the ductile metal is extruded into any crevices caused by irregularities
in the inner wall surface of the tube.
Eine Methode, um das Ende eines Inconel-Wärmeaustauscherrohrs (20) mit Hilfe eines
hohlen Inconel-Stopfens (10) in herausnehmbarer Ausführung verschließen zu können,
wobei der Stopfen über den größeren Teil des Längsabschnitts, der sich innerhalb des
Rohrs befindet, das im Vergleich zum Rohr einen geringfügig kleineren Durchmesser
besitzt, einen im wesentlichen gleichbleibenden Außendurchmesser aufweist, wobei der
genannte Stopfen ein erstes geschlossenes Ende (12) besitzt, das in die Rohre hineinragt,
sowie ein zweites offenes Ende, das so ausgeführt ist, daß es bündig mit dem Rohrende
abschließt, wobei ein Abschnitt des Stopfens in der Nähe des zweiten Endes einen geringfügig
größeren Durchmesser aufweist, um so eine leichte Preßpassung im Rohr sicherzustellen,
damit der Stopfen vor und während dessen Ausdehnung fest in seiner Position verbleibt,
wobei ein Abschnitt der Außenfläche des Stopfens ein aufgebrachtes Band aus verformbarem
Metall (18) aufweist, das eine Dicke von etwa 2-4 Milli-Inch (ca. 0,05-0,1 mm) besitzt
und entweder aus Gold, Silber oder reinem Eisen besteht, wobei die Methode folgende
Arbeitsschritte umfaßt: das Einsetzen des Stopfens in das Rohr, bis der Abschnitt
des Stopfens mit einem geringfügig größeren Durchmesser in Kontakt mit dem Rohr kommt
und mit diesem eine leichte Preßpassung eingeht, wobei das offene Ende des Stopfens
bündig mit dem Rohrende abschließt; und die anschließende Ausdehnung des Stopfens
in der Nähe des aus verformbarem Metall bestehenden Bandes in der Art, daß sich der
Stopfen so ausdehnt, daß er in eine enge Passung mit dem Rohr gebracht wird, wobei
das verformbare Metall in die aufgrund der Unregelmäßigkeiten in der Innernwandfläche
des Rohrs bestehenden Spalten gepreßt wird.
Procédé pour obturer de façon amovible l'extrémité d'un tube en Inconel pour un échangeur
de chaleur avec un bouchon creux (10) en Inconel, bouchon qui présente un diamètre
extérieur substantiellement invariable sur la plus grande portion longitudinale logée
à l'intérieur du tube de diamètre légèrement plus petit que celui du tube, ledit bouchon
ayant une première extrémité (12) fermée qui pénètre dans le tube, et une seconde
extrémité ouverte pouvant se placer au ras de l'extrémité du tube, une portion du
bouchon voisine de la seconde extrémité ayant un diamètre légèrement plus grand pour
assurer un joint à serrage léger dans le tube pour maintenir le bouchon en place avant
et pendant sa dilatation, une bande de métal ductile (18) étant plaquée sur un portion
de la surface extérieure du bouchon, ladite bande ayant une épaisseur d'environ 2-4
mils (env. 0,05-0,1 mm) et étant constituée soit d'or, soit d'argent soit encore de
fer pur, le procédé comprenant les étapes consistant à insérer le bouchon dans le
tube jusqu'à ce que la portion de diamètre légèrement plus grand du bouchon vienne
en contact avec et forme un joint à serrage léger dans le tube, l'extrémité ouverte
du bouchon se trouvant au ras de l'extrémité du tube, et ensuite à provoquer la dilatation
du bouchon au voisinage de la bande de métal ductile, de telle façon que le bouchon
soit dilaté en contact étanche avec le tube, et que le métal ductile soit extrudé
dans toute fissure résultant des irrégularités de la surface intérieure de la paroi
du tube.